Pilger mills are used for cold rolling of seamless pipes or tubing to desired dimensions and tolerances. The pilgering process is advantageously used, for example, to produce Zircaloy tubing for nuclear fuel bundle assemblies. The pilgering process consists of stepwise feeding and turning a tube between a set of tapered die rolls reciprocating over a tapered mandrel. This results in a smooth reduction of the tubing to the desired tube diameter and wall thickness.
For purposes of illustration, a pilger mill 10 is shown in FIG. 1 employing a thrust block 12 for supporting a rod 14 which extends through a feed carriage 16 to hold a mandrel die 18. In operation, an ingoing tubing 20 is pushed by the feed carriage 16 through an entry chuck 22 and between a pair of spaced rolls 24 and 26. The rolls 24 and 26 rotate as they undergo a reciprocating lateral movement as indicated by the arrow 28. The rolls have respective grooves 30 and 32 formed therein, each of which has a semicircular cross section with a groove diameter that tapers smoothly in size along the arc length on the roll circumference from the ingoing tubing diameter to the desired diameter of the finished tubing 34. As the ingoing tubing 20 advances through the reciprocating rolls, the tubing is worked to the desired diameter and degree of tolerance.
Reciprocating rotary movement is imparted to the rolls 24 and 26 by pinions mounted on the roll axle engaged with racks fixed to the machine frame (not shown). A crank drive 38 and push rod 40 are used to drive the rolls mounted on a saddle (not shown) along the racks. The two grooved rolls embrace the tubing from above and below as they roll over a predetermined length of the tubing, called the pass length. At the completion of each stroke, the entry chuck 22 and exit chuck 36 grip the tubing and rotate it a predetermined angular amount such that the entire circumference of the tubing can be worked in stepwise fashion.
The ingoing tube 20 is elongated to form the finished tubing 34 through cold rolling reduction of the tubing diameter and wall thickness between the rolls 24 and 26 and the mandrel 18. The mandrel tapers in the direction of rolling from the size of the inside diameter of the ingoing tubing 20 to the size of the inside diameter of the finished tubing 34. The condition and dimensions of the mandrel affect the amount of inner diameter reduction and the output tube quality. Key mandrel traits which result in poor quality output tubing include reverse taper, ovality, size problems, surface condition and defects.
Prior methods for addressing this problem include the use of a hand micrometer to measure the outer diameter of the mandrel at a particular location determined by eye. However, such manual measurement does not provide enough information to accurately determine if the mandrel has acceptable traits for continued use.